1. Field of the Invention
The present invention relates to a digital signal decoder for receiving or reproducing data using an error correction code having a multiple coding arrangement.
2. Description of the Prior Art
Although the procedure of recording data is not directly associated with the scope and spirit of the present invention, it will briefly be explained for ease of understanding.
The procedure starts with arranging digital data into a two-dimensional array (an lxk symbol array, where l and k are integers) with a column code encoder. Each of the k columns of the array is encoded by an error correction routine to produce mxk array codes. Then, each of the m rows are error correction encoded with a row code encoder to produce mxn array codes which are in turn modulated by a modulator prior to storage in a recording medium.
The procedure of retrieving a data from recorded signals is explained as follows. A reproduced signal from the recording medium is fed to a demodulator where it is demodulated into digital data. The digital data is transferred to a row code decoder where row code components of the data are decoded. More particularly, each of the m rows of the array data is decoded by t-tuple error correction and s-tuple error detection routines (where t and s are integers satisfying t&lt;s) to produces mxk codes. In decoding, if more than s error symbols are found in one row of the data, an erase flag is set up indicating that all of the symbols of the row have errors. The resultant erase-flag-carrying data is then fed to a column code decoder where each of the k columns of the data is decoded by u-tuple error correction and v-tuple erase correction routines (where u and v are integers) to produce lxk array data. More specifically, the column code decoder performs decoding with u-tuple error correction and v-tuple erase correction routines while identifying the location of each error symbol accompanied by an erase flag given by the row code decoder. As a result, reproduced data in which errors have been corrected will be acquired. One of such techniques of error correction is depicted in "Error control coding fundamentals and applications" (Prentice-Hall) by Shu Lin and D. J. Costello Jr.
However, the disadvantage of the foregoing technique is that if the number of error rows with erase flags is more than n+v+1, the column code decoder will fail to perform error correction, thus providing no appropriate decoding. In the common practice of the row code decoder, erase flags are assigned to all of the symbols of a corresponding row which contains one or more error symbols and hence, correct symbols of the row are also decorated by erase flags. As a result, even if one column contains less than u error symbols, it is systematically judged by the column code decoder that the column carries more than u+v+1 of error symbols and correctable error symbols will be left uncorrected. In short, the error correction in a two-dimensional array code data will be conducted without success.